getting to Atlantis on your own is very easy. You take a taxi either land or water over to Paradise Island, we took the water taxi over (then took a regular taxi on the way back.) you walk the pier for a bit by these amazingly huge yachts, you enter the hotel at the casino - which is gorgeous. There is a booth in the casino where you purchase your tickets to the Aquarium. There is a sports bar in the casino that makes really good sandwiches. After you do the Aquarium you can wander around the common area of the hotel. There were some souveniour shops just off the property. When you want to return to your ship just grab a cab.
Ok, this has been posted on other boards here. Weve stayed onsite at Atlantis several times. The place is incredible and well worth a few days stay despite the overpricing, so so food, and not so helpful staff (though there are a few good ones).
You must be a paying guest (have a room) of either the Atlantis resort or Comfort Suites (across the street) in order to tour the outside grounds (aquariums) and to use the pools, slides, and beach. I believe you can get a day room, max 4 people per room for a few hundred dollars and depending on availability. There are tours available onsite, or through the ships for around $25 pp. However, the tours will only get you into the Dig, outside aquariums and grounds. You can walk around the casino and shops on your own, even dine (if you really want to spend the money for not so great food) on your own. Wristbands are given out daily (colors change daily) to those who show a room key. Staff are often posted outside every doorway, at the entrance to the Dig, and wandering around who check for these wristbands. If you cant produce one or a room key, they will send you back inside the resort. My husband kept cutting his wristband off and constantly had to dig out his room key which he often left back in the room. We saw lots of people being told to go back inside when they couldnt come up with either. And by the way, if youre going in the winter / cool time-the pools are not heated. We were last there over Valentines Weekend and hardly anyone was in the pool because they were so cold. No sense in getting a room to use the pools/slides during that time!